International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 3, Issue 4, March 2014)
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Abstract
—
T his document gives a study case for lightning overvoltage analysis of 55MW hydro power station. This paper shows the use of ATP in lightning overvoltage analysis.
Analysis covers the selection of basic insulation level (BIL) of power station equipment’s, decision on location of surge arrester and rating of surge capacitor at generator terminal, requirement of surge arrester at generator transformer HV terminal.
Switzerland‖, the project site is located approximate
300km from Madan, Indonesia; available lightning data of
Madan is being used in the study.
The annual number of lightning strokes per km² Td ranges between 1 and 16 in Indonesia. For Peusangan, a value of 16 km-2a-1 was assumed. The keraunic level
(annual number of thunderstorm days) is 120 (equation 1).
Eq. 1
Keywords — lightning over voltage, insulation co-ordination,
ATP modelling of lightning over voltages.
I.
I NTRODUCTION
The lightning protection capability of the 150 kV overhead lines was analyzed based on the lightning distribution (figure-1). This distribution has a median of
25 kA.
Lightning over voltages are major threat for power network reliability, which causes major damage to the electrical equipment’s exposed to lightning surge.
Lightning is a non-predictable phenomenon and it is very difficult to measure the value of its current, there are several agencies available worldwide who are working for collecting lightning data of particular region. In this paper we are considering the case of a 55 MW hydro power station named as peusangan hydroelectric project as a study case which is located in Indonesia, power station configuration consist two set of 22.5 MW hydro generator turbine set, two set of three phase 26.5 MVA transformer,
150 KV switchyard, generator connected to 11KV switchgear by means of 11KV SPBD and from SPBD to transformer 11KV cable is connected. From Generator transformer to switchyard there is a 340 meter long 150KV cable. Scope of the study if limited to following –







Quantity, locations & characteristics of the Lightening
Arrestors at 150kV.
BIL of 150kV XLPE cables & Accessories.
BIL of 150kV switchyard associated accessories.
BIL of 150/11 KV, 26.5 & 25 MVA, Main transformer.
Quantity, locations & characteristics of the Lightening
Arrestors at 11kV.
Rating of the Surge Capacitor at 11kV.
BIL of 11kV, 22.5MW, Generator. f (ib)
No. of year
Figure 1: Lightning probability P(ib)
III.
S IMULATION M ETHODOLOGY
The transient simulation was done with ATP. The complete electrical system of the HEP (MV and HV part), switchyard, overhead line and unit connection is being simulated. The units were in operation (50 Hz grid voltage is considered in the transient simulation) different configurations of the plant (e.g. 1 unit on 2 lines) are being analysed. The lightning current was applied during the voltage maximum in one phase.
II.
B OUNDARY C ONDITION
Peusangan is located in an area with high lightning probability, Lightning data taken from the document published by ―world metrological organization.
A.
Fast Transient Analysis of Electrical System
The lightning performance analyses of the complete electrical system from 150kV grid to 11 kV hydro generators shall be performed for worst operating condition of the Peusangan HEP electrical system. The lightning surge study shall cover the following cases:
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International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 3, Issue 4, March 2014)



Case1: Direct stroke to phase wire - near switchyard gantry
Case2: lightning stroke to shield wire (critical flashover current)- near first tower
Case3: Back flashover- near switchyard gantry
40.0
30.0
20.0
10.0
0.0
U [kV]
B.
Computation Method
Deterministic computation, when critical current magnitude, front steepness and discharge location are selected in such a way that worse condition can appear with a very small probability. Both first negative strokes and positive strokes will be applied for the simulation and worst case obtained from the result is being considered for selection of basic insulation level (BIL).
Statistical approach, when current magnitude, front steepness and discharge location are treated as random values represented with probability distribution functions.
The results of statistical computation will be the most significant in insulation co-ordination final conclusions.
The point of lightning discharge to the earth wire causing back-flashover will be varied along the first 4 spans in front of substation and the Mean Time between
Failures (MTBF) will be estimated on the bases of latest applicable standard lightning parameter distributions for the first negative strokes.
-10.0
-20.0
-30.0
I [kA]
-40.0
-40.0 -30.0 -20.0 -10.0 0.0
10.0 20.0 30.0 40.0
Figure 2: Characteristic of MV surge arrester (U r
= 11 kV)
B.
Transmission Line Tower
Following geometry of the tower has been modeled, below the technical parameters are indicated –
IV.
I NPUT P ARAMETERS
Input parameters has been consider based on available site data in contract general specifications, although the
LPL level and ground flash density and particular data for lightning is not specified in the contract lightning protection level I has been consider to perform study case.
A.
Generator
Figure-3: Geometry of transmission tower [10]
Transmission tower is being modeled in ATP using philosophy of lossless line, in this loss line model each section of tower in represented with a calculated surge impedance, This model is recommended by CIGRE and based on travel time of transmission tower by Wachisholm,
YL Chow and KD Shrivastava.
The generator model is a standard numerical model employing Park’s transformation. The main generator data are shown below. C.
Transmission Line
U
S r r
= 11 kV
= 22.5 MW f = 50 Hz
Since no switching operations were simulated for the
MV level, the generator circuit breaker was not included, only the surge capacitor and the MV surge arrester (Ur = 11 kV). The surge arrester’s characteristic is shown in figure 2.
Overhead line (1 earth wires and 1, 3-phase
Transmission line) is being modeled with a frequency dependent J marti model, based on the geometrical data of the tower, the surge impedance as well as the insulators details. Additionally, the earth resistance of the tower was approximated with a current depending relation.
Following parameters of overhead line transmission line conductor has been used for performing simulation.
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International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 3, Issue 4, March 2014)
Table -1
Transmission line parameters
Total radius of cable in 42 mm
Configuration: Horizontal
The cable length from switchyard to Main Transformer is assumed with 340 m.
Description Value
Number of conductor in bundle
Phase conductor radius R in mm
1
10.9 mm
Resistance of conductor
Shield wire radius
Shield wire resistance
0.143 ohm/km
5mm
1.21 ohm/km (AC ate 75deg. C),
0.998 ohm/km (DC ate 20 deg. C)
241.6 For Al and 39.19 for steal, total area is 280.8 mm2.
Phase conductor effective cross section of Al conductor (mm2)
Span length 7 km from Peusangan -1 to peusangan-2 and 54km from
Peusangan-2 to grid
Spacing between towers
Surge impedance
300mtr
402 ohm
Line insulator chain length 1.2 m
Name of the conductor ACSR Hawk
Figure-4: cable model from ATP view
G.
Sheath Voltage Limiter
30.0
U [kV]
20.0
10.0
0.0
-10.0
D.
Main Transformer
The main transformer consists of a standard transformer model and a capacitance network for the winding to winding as well as winding to ground capacitance. The main data of the transformer are:
U1 = 150 kV
U2 = 11 kV
Ynd1
S = 2 x 26.5 MVA f = 50 Hz uk = 12.5 %
E.
SPBD
The SPDB is being represented with a surge impedance of 48 Ohm, and a phase velocity of 300 m/µs (standard value for air). The length was assumed with 7 m between the generator and 11 KV switchgear panel. Between switchgear panel and transformer there is a cable of
800sqmm with length 50 meter, this cable is represented with distributed parameter line of surge impedance of 45 ohm and phase velocity of 300 m/µs.
-20.0
I [kA]
-30.0
-40.0 -30.0 -20.0 -10.0 0.0 10.0 20.0 30.0 40.0
Figure-5: Characteristic of SVL (from ATP view)
Following technical details if SVL is being considered –
Ur = 6.25 kV
Uc = 5 kV
Nominal discharge current = 10 kA
Two cases is being prepare for finalization SVL location

Cable sheath earth at switchyard and connected with
SVL at transformer end

Cable sheath earth at transformer end and connected with SVL at switchyard.
Results for both the cases are being indicated in the output report accordingly recommendation is being indicated.
F.
EHV XLPE cable
The HV cable is being modelled based frequency dependent J marti model. The following data were employed:
Insulation material: XLPE, Loss factor: 0.0004
Conductor material: Copper, 240 mm² for main conductor
Insulation thickness: 23 mm
H.
Switchyard
AIS usually require a detailed model to take internal reflections into account. Due to the short element lengths only a basic line model is being employed. Phase velocity as air (300 m / µs) and the surge impedance of the switchyard was assumed with 402 Ohm similar to transmission line. The individual elements of the AIS
(feeders and busses in between) were assigned a length of
30 m.
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International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 3, Issue 4, March 2014)
Cable sealing end is modelled with capacitor with value of 0.00232 µ F.
I.
Lightning Arrester at Switchyard

Negative stroke to tower with back flash over (BFO):
80 kA, T1 = 1.8 µs, T2 = 100 µs, n (di/dt) = 6

Positive stroke to tower with back flash over (BFO):
200 kA, T1 = 3.5 µs, T2 = 100 µs, n (di/dt) = 6
The characteristic of the HV lightning arresters is shown in below figure
200
LIGHTNING CURRENT WAVE
[kA]
U [kV]
600.0
160
450.0
300.0
120
150.0
0.0
-150.0
-300.0
-450.0
-600.0
-20.0 -15.0 -10.0 -5.0
0.0
I [kA]
5.0
10.0 15.0 20.0
Figure-6: Characteristic of line arrester (from ATP view) Technical details of arrester –
Rated voltage – 144kV
MCOV – 115kV
Nominal discharge current – 10kAp
J.
Lightning Current Parameters
IEC 62305-1, Table A.1 gives the statistical values
(lognormal distribution) for lightning parameters.
Values for Lightning Protection Level I (LPL I) are the recommendations for the highest protection degree.
According to the Electro-Geometrical Model (EGM) for the overhead line, the lightning current which can hit a phase wire directly is limited to 9.5 kA (radius of lightning sphere r = 46.2 m). To take some safety margin into account, 15 kA were used in the simulation.
For the study, the following 3 wave shapes are used:

Negative direct stroke to phase wire (from EGM, 9.5 kA): 15 kA, T1 = 1.8 µs, T2 = 100 µs, n (di/dt) = 6
80
40
0
0.00
0.04
0.08
0.12
0.16
(f ile ATP_probelm.pl4; x-v ar t) c:BFO -XX0023 c:DFC -TWR2B c:CFO -Y 29
[ms] 0.20
Figure 7: Lightning current sources with 15 kA, 100 kA and 200 kA
V.
T RANSIENT S IMULATION
The numerical simulation requires a constant time step for the simulation. The upper limit for the time step is defined by the shortest line (in this case the line element of the AIS), while the lower limit is restricted by calculation speed. For the simulation, a time step of 10 ns was chosen.
Usually, a safety margin has to be taken into account. In this chapter, the calculated values are shown while a safety margin of 10 % will be applied for most of the component in the discussion.
A.
Configuration of Plant
The AIS allows the HEP to be operated in many configurations (1 & 2 generators online). Since the peak value of the transient overvoltage strongly depends on the configuration, each configuration was simulated with the 3 lightning wave shapes.
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International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 3, Issue 4, March 2014)
B.
ATP Model
500
OVERVOLTAGE AT TR. LINE TERMINATION ON SWITCHYARD (LA TERMINAL)
[kV]
400
300
200
100
0
Figure 8: ATP model of study performed
C.
Results
The results for the overvoltage in the system due to different configuration and current wave shapes are shown below. The worst conditions achieved from various cases are indicated below.
The lightning current was started as soon as the operating voltage in 1 phase was in the peak amplitude.
Since the rotational angle of the transformer is only 30°, the simulations are valid for the HV and MV level as well.
Below results has been achieved based on ATP model indicated in section B.
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1.0
0.5
0.0
-100
0 2 4 6 8
(file ATP_probelm.pl4; x-var t) v:TWR2A v:TWR2B v:TWR2C
10 12 14 [us] 16
2.5
Figure-9 -Case – Direct lightning stoke to B phase conductor
OVERVOLTAGE AT TR. LINE INSULATOR STRING
[MV]
2.0
1.5
-0.5
0 2 4 6
(f ile ATP_probelm.pl4; x-v ar t) v :TWR1A v :TWR1B v :TWR1C
8 [us]
Figure-10- Case – critical flashover current stroke to line shield wire
10

3500
[V]
3000
International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 3, Issue 4, March 2014)
OVERVOLTAGE AT GENERATOR TERMINAL OVERVOLTAGE AT GENERATOR TRANSFORMER HV
400
[kV]
350
300
2500
250
2000
200
1500
150
1000
500
6000
4500
3000
1500
0
0.00
0.03
0.06
(file ATP_probelm.pl4; x-var t) v:GEN1A v:GEN1B v:GEN1C
0.09
0.12
Figure-12-Case – Back flashover condition
OVERVOLTAGE AT GENERATOR TRANSFORMER MV
9000
[V]
7500
[ms] 0.15
100
50
5
0
20
[kV]
15
0
0.00
0.04
0.08
(file ATP_probelm.pl4; x-var t) v:MTW1A v:MTW1B v:MTW1C
0.12
0.16
[ms] 0.20
Figure-13– Back flashover condition (Sheath earthed at switchyard end)
OVERVOLTAGE AT CABLE SVL
10
0
-1500
-3000
89 90 91 92
(file ATP_probelm.pl4; x-var t) v:MTW2A v:MTW2B v:MTW2C
93 94
Figure-11-Case – Back flashover condition
95 [us] 96
-5
-10
-15
-20
0.00
0.04
(file ATP_probelm.pl4; x-var t) v:SC3C
0.08
0.12
0.16
[ms] 0.20
Figure-14 Case – Back flashover condition (Sheath earthed at transformer end)
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International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 3, Issue 4, March 2014)
D.
Lightning Performance results
The lightning performance study of the overhead line was done with the statistical distribution described in section 4. The geometrical data of the overhead line was used along with the EGM. For the BFO, the following approximation of critical current was used:
Eq. 4
BIL = 750 kV
It is recommended to connect the gantry structure and possibly first towers with switchyard grounding riser to maintain the tower foot resistance to 5 ohm.
It is recommended not to connect the sheath of the 340 m long HV cables directly to ground at the transformer terminal rather use sheath voltage limiter (SVL). Cable sheath must be directly earthed at switchyard end.
The following surge arresters shall be employed:

At the HV switchyard, 1 per feeder (Ur = 144 kV)

At the outdoor termination of EHV cable, 1 per feeder
(Ur = 144 kV)

At the HV cable shield, main transformer side (Ur to be defined by cable manufacturer)

At the generator terminal (Ur = 11 kV)
Z = 402 Ohm surge impedance of earth wire
R = 10 Ohm ground impedance of transmission line tower (assumed)
R= 5 Ohm Ground impedance of gantry structure and first transmission tower (assumed)
The critical current is calculated to 80 kA. Currents higher than this are assumed to lead to BFO at least in 1 phase.
With the lower current limit of the EGM (15 kA) and the upper limit of the BFO, 3 current ranges can be defined:
 0 … 15 kA: Currents can hit the phase wire as well as the earth wire directly
 15 kA … 80 kA: Currents can only hit the earth wire, without BFO

80 kA and above: Currents can only hit the earth wire and potentially lead to BFO
B.
Rating of Surge Capacitor on MV Level
Surge capacitor of 0.25µ F has been considered during simulation, and as per the result achieved the maximum surge at generator terminal is 14.3 KV which is well within the limit.
C.
BIL of MV and HV equipment
Table 2 shows the minimum requirements for the BIL of the HV and MV equipment.
Equipment
Table 2
Required Basic Insulation Levels
BIL
Switch Yard
750 kV
VI.
R ECOMMENDATION
Resulting from the simulations, the following design recommendations are given to guarantee the integrity of the
HV system of Peusangan HEP.
XLPE cables + accessories
Generator transformer
SPBD
750 kV
750 kV on HV level
75 kV on MV level
75 kV
A.
Surge Arrester
The numerical simulation shows that the surge arresters at the switchyard and at the cable termination (switchyard) are sufficient to provide the required overvoltage protection.
From the calculated data considering worst case of
200KA (very low probability) back flashover it is observed that the maximum value of surge at main transformer HV winding is 540KV this is well within the limit of BIL level of transformer also maximum value of surge transference at main transformer MV is 19.5 KV, therefore there is no such requirement of surge arrester at main transformer, system is well protected without surge arrester at main transformer HV termination.
Generator (*)
49 kV
VII.
S UMMERY
The transient and temporary overvoltage values for
Peusangan HEP resulting from atmospheric discharges has been examined in this study. A very detailed simulation model (operating voltage, line models, various lightning sources and worst case assumptions) was used. Therefore a safety margin of 10 % is applied to the calculated values for most of the component. The highest overvoltage values are summarized in table 3:
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Position
International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 3, Issue 4, March 2014)
Table 3
Summary of transient overvoltage values [1]
Switch Yard
Main
Transformer
HV level
Main
Transformer
MV level
Generator
Calculated overvoltage in kV
572
540
19.5
14.2
Safety margin
10 %
10 %
10 %
10 %
Result BIL BIL o.k.?
629.2 750 YES
594 750 YES
21.45 75 YES
15.62
49
(*)
YES
(*) Note: The BIL of the generator is generally calculated acc. to
IEC 60034-15 depending on the rated voltage (BIL = 4 Ur + 5 kV).
REFERENCES
[1] Insulation co-ordination –, Part 1: Definitions, principles and rules.
( IEC Std. 60071-1-2006)
[2] IEC 60071-2, Insulation co-ordination – Part 2: Application Guide. (
Third Edition, 1996)
[3] Eriksson, A. J., ―The Incidence of Lightning Strikes to Power
Lines‖, (IEEE Transactions on Power Delivery, vol. 2, pp. 859-870,
July 1987).
[4] H. K. Hoidalen, ATPDraw for Windows version 5.5, 2010.
[5] IEEE Design Guide for Improving the Lightning Performance of
Transmission Lines, (IEEE Std. 1243-1997)
[6] Surge arresters - Part 8: Metal-oxide surge arresters with external series gap (EGLA) for overhead transmission and distribution lines of a.c. systems above 1 kV, ( IEC 60099-8 Ed. 1.0, 2009).
[7] J. A. Martinez and F. C. Aranda, "Tower Modeling for Lightning
Analysis of Overhead Transmission Line," ( Proceedings of 2005
IEEE Power Engineering Society General Meeting, Vol. 2, June 12-
16, 2005, p.p.1212- 1217).
[8] IEEE Standard for Insulation Coordination—Definitions, Principles, and Rules, ( IEEE Std. 1313.1-1996).
[9] M. Ishii, T. Kawamura, T. Kouno, E. Ohsaki, K. Shiokawa, K.
Murotani, and T. Higuchi, ―Multistory transmission tower model for lightning surge analysis‖, ( IEEE Trans. Power Delivery, vol. 6, pp.
1327–1335, July 1991).
[10] Prof. ir. L. van der Sluis, ir. S.W.H. de Haan , Dr.ir. M. Popov, modelling of 150kV Indonesian power system for voltage stability.
Based on result achieved it is observed that there is no requirement of surge arrester at Main transformer HV terminal.
The statistical lightning performance estimation of the overhead line shows that annually 0.25 atmospheric discharges will hit a phase wire directly while 58.4 discharges will hit the earth wire leading to back flash over.
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